Pulse-Amplitude Modulation 4-Level (PAM-4) signaling is being proposed for future graphic memory interfaces as a way to increase communication bandwidth between memory and other system components, such as central processing units (CPUs) and graphics processing units (GPUs). Due to its use of >2 voltage level signals, PAM-4 is more susceptible to noise introduced from ISI and crosstalk than are binary voltage level techniques such as PAM-2.
In telecommunication, inter-symbol interference (ISI) is a form of distortion of a signal in which one symbol interferes with subsequent symbols. This is a disruptive phenomenon as the previous symbols have a similar effect as noise, thus making the communication less reliable.
Crosstalk is any phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel. Crosstalk is usually caused by undesired capacitive, inductive, or conductive coupling from one circuit or channel to another.
There is therefore a need for encoding that reduce the effects of ISI and crosstalk in PAM-4 systems.